Series transformer fast commissioning system and method

Abstract

The application discloses a series transformer fast input system and method, which comprises a power grid, a load, a series transformer, a bypass switch, a local energy source, an inverter and a filter circuit. The series transformer is connected in a same-name end reverse connection mode. The secondary winding of the series transformer is connected in series in a main loop between the power grid and the load. The primary winding of the series transformer is connected with the output end of the filter circuit. The bypass switch is arranged between the power grid and the load. The direct current side of the inverter is connected with the local energy source. The alternating current side of the inverter is connected with the primary winding of the series transformer through the filter circuit. The application realizes fast opening of the switch under the condition of zero current by controlling the inverter output current and the load current in reverse, and connects the transformer into the main loop in series, thereby greatly improving the response speed of voltage compensation.

Description

Technical Field

[0001] This invention relates to a system and method for rapid connection of series transformers, belonging to the field of power system power quality management technology. Background Technology

[0002] Voltage sags are a common power quality problem in power systems, mainly caused by various short-circuit faults. The supply voltage drops rapidly to 10%–90% of the rated voltage within a short period, lasting from 10 ms to 1 minute. Due to the short duration of voltage sags, typically within 100 ms, extremely high demands are placed on the response speed of mitigation devices.

[0003] With the development of power electronics technology, various voltage sag mitigation devices are now in use. Solid-state transfer switches (SSTS) have two independent power supplies, quickly switching to the backup power supply during voltage disturbances. Uninterruptible power supplies (UPS) can maintain a constant voltage on the user side during voltage sags, but their complex structure and high cost for high-power applications are significant. Unified power quality controllers (UPQC) consist of series and parallel active power filters, and are costly, complex in structure, and difficult to control. Dynamic voltage restorers (DVRs) inject compensation voltage through a series transformer in the load circuit, offering fast response and good dynamic performance, making them a widely used voltage sag mitigation solution.

[0004] However, the commissioning of series transformers faces a key technical challenge. During normal operation, if the transformer is always connected in series in the circuit, a short-circuit fault on the grid side requires rapid disconnection of the switch to cut off the original branch circuit. Current technology typically uses thyristors as switching devices, in conjunction with mechanical switches (circuit breakers), to enable the transformer bypass. The disconnection speed directly determines the commissioning speed of the series transformer, thus affecting the response time of voltage compensation. Using this method, when the mechanical switch disconnects, a large load current still flows through the switch contacts at the moment of disconnection, generating an arc. The arc must be extinguished after the current crosses zero, with an arc extinguishing time of approximately 40-60 ms. Furthermore, the short-circuit current still needs to flow through the thyristor; thyristors with a current carrying capacity of tens of thousands of amperes are expensive, significantly increasing system costs.

[0005] Considering that voltage sag events typically last only 10ms to 100ms, the aforementioned 20ms bypass disconnection time will severely impact the compensation effect and may even lead to compensation failure. Therefore, how to achieve rapid commissioning of series transformers and shorten the commissioning time of series transformers for compensation is a key technical problem that urgently needs to be solved in the field of voltage sag mitigation. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a series transformer fast connection system. This system controls the inverter output current to be reversed with the load current, and realizes fast switching under zero current conditions, so as to quickly connect the transformer in series to the main circuit, which greatly improves the response speed of voltage compensation.

[0007] Another object of the present invention is to provide a method for rapid connection of series transformers, which is based on the above-mentioned rapid connection system for series transformers.

[0008] The objective of this invention can be achieved by adopting the following technical solutions:

[0009] A series transformer fast-connection system includes a power grid, a load, a series transformer, a bypass switch, a local energy source, an inverter, and a filter circuit. The series transformer is connected in reverse connection mode with the same-name terminals. The secondary winding of the series transformer is connected in series to the main circuit between the power grid and the load. The primary winding of the series transformer is connected to the output terminal of the filter circuit. The bypass switch is located between the power grid and the load. The DC side of the inverter is connected to the local energy source, and the AC side of the inverter is connected to the primary winding of the series transformer through the filter circuit.

[0010] Furthermore, the power grid output voltage U s Power supply current i s The current flows through the secondary winding of the series transformer to supply power to the load, and the voltage across the load is U. L The load current is i L The series transformer has a turns ratio of k, a primary winding current of i, and a voltage across the primary winding of U.

[0011] Furthermore, the inverter is a voltage-source H-bridge inverter, including fully controlled switching devices V1, V2, V3, and V4. Fully controlled switching devices V1 and V2 are connected in series to form the first bridge arm, and fully controlled switching devices V3 and V4 are connected in series to form the second bridge arm. The first bridge arm and the second bridge arm are connected in parallel to the two ends of the DC bus capacitor. The AC output terminal of the inverter is taken from the midpoint of the first bridge arm and the midpoint of the second bridge arm.

[0012] Furthermore, the filtering circuit includes a filtering inductor and a filtering capacitor. The filtering inductor is connected in series between the AC output terminal of the inverter and the secondary winding of the series transformer, and the filtering capacitor is connected in parallel across the secondary winding of the series transformer.

[0013] Another objective of this invention can be achieved by adopting the following technical solution:

[0014] A method for rapid connection of a series transformer, based on the aforementioned rapid connection system for series transformers, the method comprising:

[0015] The system is in normal operation, acquiring the instantaneous value signal of the grid voltage in real time, processing it and extracting phase information, and simultaneously acquiring the load current to monitor the effective value of the grid voltage in real time, detecting and judging whether a voltage sag event has occurred.

[0016] After detecting a voltage sag event, the inverter is controlled to enter the current control mode. Based on the turns ratio of the series transformer and the current load current, the inverter output current is controlled by taking advantage of the reverse connection of the same-name terminals of the transformer. This makes the secondary current of the transformer equal in magnitude and opposite in direction to the load current, thereby canceling the current flowing through the bypass switch to 0 and realizing the zero-current breaking condition of the bypass switch.

[0017] The current on the bypass switch is continuously monitored. When the current drops below the preset zero current threshold, and the zero current interruption condition is confirmed, a tripping command is sent to the bypass switch to control the bypass switch to quickly disconnect. The series transformer is then formally connected in series in the main circuit between the power supply side and the load side.

[0018] The inverter switches from current control mode to other control modes, and, in conjunction with the topological characteristics of the series transformer, adjusts and controls the electrical quantities of the main circuit to achieve the target functions required after the series transformer is put into operation, and the system enters a new stable operating state.

[0019] Furthermore, the real-time acquisition of the instantaneous value signal of the power grid voltage includes:

[0020] The grid voltage is sampled using a voltage transformer to obtain the instantaneous value signal of the grid voltage. Let the instantaneous value of the grid voltage be:

[0021] ;

[0022] Among them, U m ω is the grid voltage amplitude, ω is the grid angular frequency, and φ is the initial phase of the grid voltage.

[0023] Furthermore, the real-time monitoring of the effective value of the grid voltage, and the detection and judgment of whether a voltage sag event has occurred, includes:

[0024] Based on the instantaneous value of the grid voltage, the effective value U of the grid voltage is calculated using a discretization method. s :

[0025] ;

[0026] Where N is the number of sampling points in one period, u s (i) represents the instantaneous voltage value at the i-th sampling point;

[0027] According to the effective value of the grid voltage U s Calculate the voltage sag depth d:

[0028] ;

[0029] Among them, U N This is the rated voltage of the power grid;

[0030] The voltage sag depth d is compared with the preset voltage sag threshold d. th When comparing, when d > d th And the duration exceeds the temporary landing confirmation time t confirm Subsequently, a voltage dip event was officially determined to have occurred.

[0031] Furthermore, the inverter is controlled to enter current control mode. Based on the turns ratio of the series transformer and the current load current, and utilizing the characteristic of reverse connection of the transformer's same-name terminals, the inverter output current is controlled to make the secondary current of the transformer equal in magnitude and opposite in direction to the load current. This cancels out the current flowing through the bypass switch to zero, achieving the zero-current breaking condition of the bypass switch, including:

[0032] During normal operation, the bypass switch is closed, and the load current i L The current flows to the load through the bypass switch, let the current on the bypass switch K be i. K The primary current of the series transformer is i1, and the secondary current is i2. According to Kirchhoff's current law, the node current relationship is as follows:

[0033] ;

[0034] When the series transformers are connected with the same-name terminals reversed, and the inverter output current is i, the inverter output is negative, the node current relationship becomes:

[0035] ;

[0036] Based on the transformer turns ratio, the relationship between the primary current i1 and the secondary current i2 of a series transformer is as follows:

[0037] ;

[0038] Substituting the relationship between the primary current i1 and the secondary current i2 of the series transformer into the node current relationship, we obtain the current on the bypass switch:

[0039] ;

[0040] When controlling the inverter output current hour:

[0041] ;

[0042] At this point, the current on the bypass switch drops to zero, satisfying the zero-current interruption condition;

[0043] The inverter is controlled by current based on the turns ratio k of the series transformer and the current load current i. L Calculate the target output value required by the inverter:

[0044] ;

[0045] in, This is the command value for the inverter output current. For reference, the inverter is driven by a PI closed-loop current controller to track the target value in real time, ensuring that the inverter output current accurately follows the dynamic changes of the load current and achieves continuous cancellation of the bypass switch current.

[0046] In current control mode, the inverter's output voltage U is determined by the transformer leakage reactance and the grid voltage. Let the transformer primary leakage reactance be X. T Then the inverter output voltage is:

[0047] ;

[0048] Continuously monitor the current amplitude i on the bypass switch K The current amplitude i K With the preset zero current threshold i th Perform real-time comparisons when |i K | th When it is determined that the bypass switch K has reached the zero-current breaking condition, the control system outputs a trip enable signal, allowing subsequent breaking operations to be performed; if |i K |>i th If the current control mode is maintained, the current will continue to decrease until the current is further reduced until |i K | th until.

[0049] Furthermore, the step of issuing a trip command to the bypass switch to control the bypass switch to quickly disconnect, and formally connecting the series transformer in series into the main circuit between the power supply side and the load side, includes:

[0050] Send a trip command to the drive circuit of the bypass switch;

[0051] The bypass switch performs a disconnection action after receiving a trip command;

[0052] After the bypass switch is disconnected, the secondary winding of the series transformer is no longer short-circuited. The series transformer is officially connected in series to the main circuit between the power supply side and the load side. The system topology is switched from normal bypass operation to series compensation operation. After synchronous detection confirms that the transformer has been normally put into operation and the main circuit topology switch is successful, a signal indicating that the transformer has been put into operation is sent to the upper-level controller. ​​

[0053] Furthermore, the inverter switches from current control mode to other control modes, and, in conjunction with the topology characteristics of the series transformer, adjusts and controls the electrical quantities of the main circuit to achieve the target functions required after the series transformer is put into operation, including:

[0054] The inverter switches from current control mode to target control mode using a soft-switching strategy. The output value at the end of the current control mode is used as the initial setpoint for the target control mode, ensuring the inverter output voltage U is maintained during the switching process. inv No continuous mutations;

[0055] After switching to the target control mode, the voltage analysis of the main circuit is performed using the topological characteristic of reverse connection of the same-name terminals of the series transformer. The load-side voltage U L With the effective value of the grid voltage U s Inverter output voltage U inv The following relationship must be satisfied:

[0056] ;

[0057] When it is necessary to compensate the load-side voltage to the target value Then, solve for the required output voltage setpoint of the inverter:

[0058] ;

[0059] by As the command value for the inverter output voltage, the inverter output is driven in real time by the closed-loop voltage controller, so that the load-side voltage U... L Stable tracking target value ;

[0060] During operation in the target control mode, the effective value of the grid voltage U is continuously monitored. S The load-side electrical quantities are measured, and when the triggering conditions disappear and the power grid returns to normal, the inverter output is controlled to return to zero smoothly. Then, the bypass switch is closed to re-short-circuit the secondary winding of the series transformer.

[0061] The present invention has the following advantages over the prior art:

[0062] 1. This invention achieves current transfer through current control mode, which enables the bypass switch to disconnect under zero current conditions, completely eliminating the arc problem and shortening the transformer activation time from 40-60ms in the traditional scheme to less than 5ms, thus greatly improving the response speed of voltage compensation.

[0063] 2. This invention adopts zero-current interruption technology, and the bypass switch does not need to withstand arc extinguishing pressure. Ordinary circuit breakers or switching devices can be used, and there is no need to configure expensive high-capacity thyristors, which significantly reduces the system cost.

[0064] 3. During the current control mode, the inverter output voltage is close to the grid voltage and the output current is at the level of normal load current. Neither the inverter nor the transformer needs to withstand short-circuit current impact, thus reducing the equipment capacity requirements. Attached Figure Description

[0065] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0066] Figure 1 This is a topology diagram of a series transformer rapid connection system according to an embodiment of the present invention.

[0067] Figure 2 This is a flowchart of a rapid connection method for a series transformer according to an embodiment of the present invention.

[0068] Figure 3 This is a schematic diagram of the Matlab / Simulink simulation model of an embodiment of the present invention.

[0069] Figure 4 This is a bypass switch circuit diagram according to an embodiment of the present invention. Detailed Implementation

[0070] To better understand the invention, various aspects of the invention will be described in more detail with reference to the accompanying drawings. It should be understood that these detailed descriptions are merely illustrative of exemplary embodiments of the invention and are not intended to limit the scope of the invention in any way. Throughout the specification, the same reference numerals refer to the same elements. The expression "and / or" includes any and all combinations of one or more of the associated listed items.

[0071] In the accompanying drawings, the size, dimensions, and shapes of the elements have been slightly adjusted for ease of illustration. The drawings are for illustrative purposes only and are not strictly to scale. As used herein, the terms “approximately,” “about,” and similar terms are used to indicate approximation, not degree, and are intended to illustrate inherent deviations in measured or calculated values ​​that will be recognized by those skilled in the art. Furthermore, the order in which the steps are described in this invention does not necessarily indicate the order in which these steps occur in actual operation, unless otherwise expressly defined or deduced from the context.

[0072] It should also be understood that expressions such as "comprising," "including," "having," "containing," and / or "comprising" are open-ended rather than closed-ended expressions in this specification, indicating the presence of the stated features, elements, and / or components, but not excluding the presence of one or more other features, elements, components, and / or combinations thereof. Furthermore, when expressions such as "at least one of..." appear after a list of listed features, they modify the entire list of features, not just individual elements in the list. Additionally, when describing embodiments of the invention, the word "may" is used to mean "one or more embodiments of the invention." And the term "exemplary" is intended to refer to examples or illustrations.

[0073] Unless otherwise specified, all terms used herein (including engineering and technical terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that, unless expressly stated herein, terms defined in common dictionaries shall be interpreted as having the meaning consistent with their meaning in the context of the relevant art, and not in an idealized or overly formalized sense.

[0074] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0075] Example:

[0076] like Figure 1 As shown, this embodiment provides a series transformer fast connection system, which includes a power grid, a load, a series transformer T, a bypass switch K, a local energy source, an inverter, and a filter circuit.

[0077] Grid output power voltage U s Power supply current i s The current flows through the secondary winding of the series transformer to supply power to the load, and the voltage across the load is U. L The load current is i L The turns ratio of the series transformer is k, where the value of k is determined according to the required compensation depth, and is generally taken as 5~10. The primary winding current of the series transformer is i, and the voltage across the primary winding is U.

[0078] The series transformer T is connected in reverse connection with the same terminal, so that the inverter output voltage is superimposed or subtracted from the grid voltage after being transformed by the transformer, thereby compensating for the load voltage; the secondary winding of the series transformer T is connected in series to the main circuit between the grid and the load, and the primary winding of the series transformer is connected to the output terminal of the filter circuit.

[0079] The DC side of the inverter is connected to the local energy source, and the AC side of the inverter is connected to the primary winding of the series transformer through a filter circuit. The inverter is a voltage-source inverter, which includes multiple fully controlled switching devices and their anti-parallel diodes.

[0080] Furthermore, the inverter is a voltage-source H-bridge inverter, including fully controlled switching devices V1, V2, V3, and V4. Fully controlled switching devices V1 and V2 are connected in series to form the first bridge arm, and fully controlled switching devices V3 and V4 are connected in series to form the second bridge arm. The first bridge arm and the second bridge arm are connected in parallel to the two ends of the DC bus capacitor. The AC output terminal of the inverter is taken from the midpoint of the first bridge arm and the midpoint of the second bridge arm.

[0081] Furthermore, the fully controlled switching devices V1, V2, V3, and V4 are all insulated gate bipolar transistors (IGBTs), and each IGBT has a freewheeling diode connected in anti-parallel.

[0082] Furthermore, the filter circuit includes a filter inductor L f and filter capacitor C f Filter inductor L f The filter capacitor C is connected in series between the AC output terminal of the inverter and the secondary winding of the series transformer T. f It is connected in parallel to the two ends of the secondary winding of the series transformer T.

[0083] Furthermore, the local energy source is an energy storage unit, a supercapacitor, a generator, and a boost rectifier module.

[0084] like Figure 2 As shown, this embodiment also provides a method for rapid connection of a series transformer. Before the bypass switch K is opened, this method first controls the inverter to enter current control mode and outputs current. The load current i is converted using the electromagnetic coupling relationship of the series transformer T. L The current is transferred from the branch circuit to the main circuit, causing the current on the bypass switch K to drop to near zero; under zero current conditions, the bypass switch K is controlled to open, eliminating the arc and achieving rapid interruption; after the bypass switch K is opened, the inverter immediately switches to voltage control mode and outputs a compensation voltage to compensate the load-side voltage to the target value, which includes the following steps:

[0085] S1. The system is in normal operation, acquiring the instantaneous value signal of the grid voltage in real time, processing it and extracting phase information, and simultaneously acquiring the load current i. L Real-time monitoring of the effective value of the grid voltage to detect and determine whether a voltage sag event has occurred includes the following steps:

[0086] S11. With the bypass switch K closed, the secondary winding of the series transformer T is short-circuited, and the system operates normally.

[0087] S12. Real-time acquisition of the instantaneous value signal of the grid voltage: The grid voltage is sampled through a voltage transformer to obtain the instantaneous value signal of the grid voltage. Let the instantaneous value of the grid voltage be:

[0088] ;

[0089] Among them, U m ω is the grid voltage amplitude, ω is the grid angular frequency, and φ is the initial phase of the grid voltage.

[0090] S13. Based on the instantaneous value of the grid voltage, calculate the effective value U of the grid voltage using a discretization method. s :

[0091] ;

[0092] Where N is the number of sampling points in one period, u s (i) represents the instantaneous voltage value at the i-th sampling point. Through the above calculation, the accurate effective value of the grid voltage can be obtained as the basis for subsequent voltage sag detection and judgment. In this embodiment, the sampling frequency is 20kHz, so N = 400.

[0093] S14. Calculate the voltage sag depth based on the effective value of the grid voltage U. s Calculate the voltage sag depth d:

[0094] ;

[0095] Among them, U N The voltage sag depth d represents the degree to which the grid voltage deviates from its rated value. The larger the d value, the more severe the voltage drop.

[0096] While determining the voltage sag, the current load current i is collected. L The amplitude and phase information are used as target reference values ​​for subsequent current control modes.

[0097] S2. Upon detecting a voltage sag event, the inverter is controlled to enter current control mode, based on the turns ratio k of the series transformer T and the current load current i. L By utilizing the characteristic of reverse connection of the same-name terminals of the transformer, the output current -i of the inverter is controlled. L / k makes the secondary current of the transformer equal in magnitude and opposite in direction to the load current, thereby canceling the current flowing through the bypass switch to 0, achieving the zero-current breaking condition of the bypass switch. The specific steps include:

[0098] S21. Current Transfer Principle: During normal operation, the bypass switch K is closed, and the load current i LThe current flows to the load through bypass switch K, let the current on bypass switch K be i. K The primary current of the transformer is i1, and the secondary current is i2. According to Kirchhoff's current law, the node current relationship is as follows:

[0099] ;

[0100] The series transformer T is connected with its terminals reversed. When the inverter outputs current i, the inverter output is negative, and the node current relationship becomes:

[0101] ;

[0102] Based on the transformer turns ratio, the relationship between the primary current i1 and the secondary current i2 of a series transformer is as follows:

[0103]

[0104] Substituting the relationship between the primary current i1 and the secondary current i2 of the series transformer into the node current relationship, we obtain the current on the bypass switch:

[0105] ;

[0106] When controlling the inverter output current hour:

[0107] ;

[0108] At this point, the current on the bypass switch drops to zero, satisfying the zero-current interruption condition.

[0109] S22. Calculate the target current value and perform current control on the inverter based on the turns ratio k of the series transformer T and the current load current i. L Calculate the target output value required by the inverter:

[0110] ;

[0111] in, This is the command value for the inverter output current. For reference, the inverter is driven by a PI closed-loop current controller to track the target value in real time, ensuring that the inverter output current accurately follows the dynamic changes of the load current and achieves continuous cancellation of the bypass switch current.

[0112] S23. Determine the inverter output voltage. In current control mode, the inverter output voltage U is determined by the transformer leakage reactance and the grid voltage. Let the transformer primary leakage reactance be X. T Then the inverter output voltage is:

[0113] ;

[0114] Because the transformer leakage inductance is small, and the current i is on the order of normal load current, X T The ×i term is much smaller than the grid voltage, so the inverter output voltage is approximately equal to the grid voltage. The inverter does not need to withstand large voltage stress, which effectively reduces the inverter's capacity requirements and device withstand voltage requirements, and improves the system's economy and reliability.

[0115] S24. Confirm the zero-current condition and continuously monitor the current amplitude i on the bypass switch. K The current amplitude i K With the preset zero current threshold i th Perform real-time comparisons when |i K | th When it is determined that the bypass switch K has reached the zero-current breaking condition, the control system outputs a trip enable signal, allowing subsequent breaking operations to be performed; if |i K |>i th If the current control mode is maintained, the current will continue to decrease until the current is further reduced until |i K | th Up to; where the zero current threshold i th The circuit breaker K is pre-set according to its rated breaking capacity and pre-arc current requirements to ensure that the switch can achieve arc-free or low-arc breaking within the safe current range and meet the zero-current breaking condition.

[0116] S3. Continuously monitor the current on the bypass switch K. When the current drops below the preset zero current threshold, and the zero current interruption condition is confirmed, send a tripping command to the bypass switch K to control it to quickly disconnect. The series transformer T is then formally connected in series in the main circuit between the power supply side and the load side. This includes the following steps:

[0117] S31. Confirm that the current amplitude on the bypass switch K satisfies |i K | th Immediately afterwards, a tripping command is sent to the drive circuit of the bypass switch K. To prevent the switch from malfunctioning before the current has been sufficiently reduced due to command delay or execution jitter, the control system must continuously confirm that the zero-current interruption condition is stably established within the set confirmation time window before issuing the tripping command. Only after confirmation can the tripping action be triggered to ensure the safety and reliability of the interruption process.

[0118] S32. After receiving the tripping command, the bypass switch K performs a disconnection action because the current flowing through the bypass switch K has dropped to the zero current threshold i. th ​​​Below, when the contacts separate at the moment of disconnection, there is basically no electric arc generated. There is no need to go through the process of arc extinguishing and dielectric strength recovery. The entire disconnection process does not need to wait for the power frequency current to naturally cross zero. The bypass switch K can complete the reliable disconnection action in a very short time.

[0119] S33. After the bypass switch K is disconnected, the secondary winding of the series transformer T is no longer short-circuited. The series transformer T is officially connected in series to the main circuit between the power supply side and the load side. The system topology is switched from normal bypass operation to series compensation operation. The main circuit current and the voltage across the transformer are detected synchronously. After confirming that the transformer has been normally connected and the main circuit topology has been successfully switched, a connection completion signal is sent to the upper-level controller to provide a trigger basis for the subsequent control mode switching.

[0120] Compared to traditional mechanical switches, which carry hundreds of amperes of current when disconnected and require waiting for the power frequency current to cross zero and the arc to be extinguished (taking 20-60ms), this embodiment achieves zero-current interruption through current transfer. From voltage sag detection (2ms) to current control mode establishment (2ms) and then to switch disconnection (3ms), the total connection time is approximately 5-7ms, which is more than 75% shorter than the traditional solution. This significantly improves the system's rapid response capability and effectively reduces the impact on power quality caused by connection delays on the load side.

[0121] S4. The inverter switches from current control mode to other control modes, and, in conjunction with the topology characteristics of the series transformer, adjusts and controls the electrical quantities of the main circuit to achieve the target functions required after the series transformer is put into operation. The system then enters a new stable operating state, which specifically includes the following steps:

[0122] S41. After detecting that the bypass switch K has been reliably disconnected, the inverter immediately switches from the current control mode to the target control mode (such as the voltage control mode). The switching process adopts a soft switching strategy, using the output quantity of the final state of the current control mode as the initial setpoint of the target control mode, ensuring that the inverter output voltage U is within the instant of switching. inv Continuous operation without sudden changes avoids electrical shocks to the main circuit.

[0123] S42. Electrical quantity regulation based on transformer topology: After switching to the target control mode, utilize the topology characteristic of reverse connection of the same-name terminals of the series transformer T to perform voltage analysis on the main circuit, and the load-side voltage U... L With the effective value of the grid voltage U s Inverter output voltage U inv The following relationship must be satisfied:

[0124] ;

[0125] When it is necessary to compensate the load-side voltage to the target value Then, the required output voltage setpoint of the inverter can be solved from the above formula.

[0126] ;

[0127] by As the command value for the inverter output voltage, the closed-loop voltage controller drives the inverter output in real time, causing the load-side voltage U to... L Stable tracking target value Due to the reverse connection of the transformer's same-name terminals, the compensation voltage is shared by the grid-side winding voltage and the inverter output voltage. The inverter does not need to bear the entire compensation voltage independently, which effectively reduces the inverter's capacity requirements.

[0128] S43. During the operation of the target control mode, continuously monitor the effective value of the grid voltage U. s The load-side electrical quantities are measured; when the triggering conditions disappear and the power grid returns to normal, the inverter output is controlled to return to zero smoothly. Then, the bypass switch K is closed to re-short-circuit the secondary winding of the series transformer T, the system topology is switched back to normal bypass operation, and the inverter re-enters standby charging mode, completing the entire control process for the rapid commissioning of the series transformer.

[0129] To verify the feasibility of the series transformer rapid connection system and method in this embodiment, a corresponding simulation model was built on the Matlab / Simulink simulation platform, such as... Figure 3 As shown, the specific parameter settings are as follows: rated grid voltage 220V, frequency 50Hz; transformer turns ratio k=2; filter inductor L f =1mH, filter capacitor C f =800μF; the load is an inductive load with a rated power of 5kW and a load current of approximately 20A; the inverter has a rated power of 1.5kW; the zero current threshold is ith=2A.

[0130] like Figure 4 As shown, when a voltage dip occurs in the grid voltage, this embodiment can detect the voltage dip within 0.5ms and enter the current control mode to control the inverter output current i=i L / k≈10.4A, after being boosted by the transformer, the current of the bypass switch is finally reduced to below 2A, achieving the condition of zero current shutdown. The voltage fluctuation on the load side is less than 5% throughout the entire process, meeting the operating requirements of sensitive loads.

[0131] It should be noted that the above simulation results are based on an idealized device model and control environment. In actual operating conditions, factors such as communication delay, sampling and processing time, switching device action delay, and electromagnetic interference will prolong the switch turn-off time. In practical applications, the total response time from detecting a voltage dip to completing the zero-current interruption of the bypass switch is expected to be approximately 5ms, which is significantly better than the 20-60ms response time of traditional mechanical switch schemes.

[0132] It should be noted that, in this invention, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0133] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A rapid connection system for a series transformer, characterized in that, The system includes a power grid, a load, a series transformer, a bypass switch, a local energy source, an inverter, and a filter circuit. The series transformer is connected in reverse connection with the same-name terminals. The secondary winding of the series transformer is connected in series to the main circuit between the power grid and the load. The primary winding of the series transformer is connected to the output terminal of the filter circuit. The bypass switch is located between the power grid and the load. The DC side of the inverter is connected to the local energy source, and the AC side of the inverter is connected to the primary winding of the series transformer through the filter circuit.

2. The series transformer rapid connection system according to claim 1, characterized in that, The power grid output voltage U s Power supply current i s The current flows through the secondary winding of the series transformer to supply power to the load, and the voltage across the load is U. L The load current is i L The series transformer has a turns ratio of k, a primary winding current of i, and a voltage across the primary winding of U.

3. The series transformer rapid connection system according to claim 1, characterized in that, The inverter is a voltage-source H-bridge inverter, including fully controlled switching devices V1, V2, V3, and V4. Fully controlled switching devices V1 and V2 are connected in series to form the first bridge arm, and fully controlled switching devices V3 and V4 are connected in series to form the second bridge arm. The first bridge arm and the second bridge arm are connected in parallel to the two ends of the DC bus capacitor. The AC output terminal of the inverter is taken from the midpoint of the first bridge arm and the midpoint of the second bridge arm.

4. The series transformer fast connection system according to claim 1, characterized in that, The filtering circuit includes a filtering inductor and a filtering capacitor. The filtering inductor is connected in series between the AC output terminal of the inverter and the secondary winding of the series transformer, and the filtering capacitor is connected in parallel across the secondary winding of the series transformer.

5. A method for rapid connection of a series transformer, implemented based on the rapid connection system for a series transformer as described in any one of claims 1-4, characterized in that, The method includes: The system is in normal operation, acquiring instantaneous grid voltage signals in real time, processing them and extracting phase information, while also acquiring load current, monitoring the effective value of grid voltage in real time, and detecting and determining whether a voltage sag event has occurred. After detecting a voltage sag event, the inverter is controlled to enter the current control mode. Based on the turns ratio of the series transformer and the current load current, the inverter output current is controlled by taking advantage of the reverse connection of the same-name terminals of the transformer. This makes the secondary current of the transformer equal in magnitude and opposite in direction to the load current, thereby canceling the current flowing through the bypass switch to 0 and realizing the zero-current breaking condition of the bypass switch. The current on the bypass switch is continuously monitored. When the current drops below the preset zero current threshold, and the zero current interruption condition is confirmed, a tripping command is sent to the bypass switch to control the bypass switch to quickly disconnect. The series transformer is then formally connected in series in the main circuit between the power supply side and the load side. The inverter switches from current control mode to other control modes, and, in conjunction with the topological characteristics of the series transformer, adjusts and controls the electrical quantities of the main circuit to achieve the target functions required after the series transformer is put into operation, and the system enters a new stable operating state.

6. The method for rapid connection of a series transformer according to claim 5, characterized in that, The real-time acquisition of instantaneous grid voltage signals includes: The grid voltage is sampled using a voltage transformer to obtain the instantaneous value signal of the grid voltage. Let the instantaneous value of the grid voltage be: ； Among them, U m ω is the grid voltage amplitude, ω is the grid angular frequency, and φ is the initial phase of the grid voltage.

7. The method for rapid connection of a series transformer according to claim 5, characterized in that, The real-time monitoring of the effective value of the power grid voltage, and the detection and determination of whether a voltage sag event has occurred, includes: Based on the instantaneous value of the grid voltage, the effective value U of the grid voltage is calculated using a discretization method. s : ； Where N is the number of sampling points in one period, u s (i) represents the instantaneous voltage value at the i-th sampling point; According to the effective value of the grid voltage U s Calculate the voltage sag depth d: ； Among them, U N This is the rated voltage of the power grid; The voltage sag depth d is compared with the preset voltage sag threshold d. th When comparing, when d > d th And the duration exceeds the temporary landing confirmation time t confirm Subsequently, a voltage dip event was officially determined to have occurred.

8. The method for rapid connection of a series transformer according to claim 5, characterized in that, The inverter is controlled to enter current control mode. Based on the turns ratio of the series transformer and the current load current, and utilizing the characteristic of reverse connection of the transformer's same-name terminals, the inverter output current is controlled to make the secondary current of the transformer equal in magnitude and opposite in direction to the load current. This cancels out the current flowing through the bypass switch to zero, achieving the zero-current breaking condition of the bypass switch, including: During normal operation, the bypass switch is closed, and the load current i L The current flows to the load through the bypass switch, let the current on the bypass switch K be i. K The primary current of the series transformer is i1, and the secondary current is i2. According to Kirchhoff's current law, the node current relationship is as follows: ； When the series transformers are connected with the same-name terminals reversed, and the inverter output current is i, the inverter output is negative, the node current relationship becomes: ； Based on the transformer turns ratio, the relationship between the primary current i1 and the secondary current i2 of a series transformer is as follows: ； Substituting the relationship between the primary current i1 and the secondary current i2 of the series transformer into the node current relationship, we obtain the current on the bypass switch: ； When controlling the inverter output current hour: ； At this point, the current on the bypass switch drops to zero, satisfying the zero-current interruption condition; The inverter is controlled by current based on the turns ratio k of the series transformer and the current load current i. L Calculate the target output value required by the inverter: ； in, This is the command value for the inverter output current. For reference, the inverter is driven by a PI closed-loop current controller to track the target value in real time, ensuring that the inverter output current accurately follows the dynamic changes of the load current and achieves continuous cancellation of the bypass switch current. In current control mode, the inverter's output voltage U is determined by the transformer leakage reactance and the grid voltage. Let the transformer primary leakage reactance be X. T Then the inverter output voltage is: ； Continuously monitor the current amplitude i on the bypass switch K The current amplitude i K With the preset zero current threshold i th Perform real-time comparisons when |i K | th When it is determined that the bypass switch K has reached the zero-current breaking condition, the control system outputs a trip enable signal, allowing subsequent breaking operations to be performed; if |i K |>i th If the current control mode is maintained, the current will continue to decrease until the current is further reduced until |i K | th until.​​ 9. The method for rapid connection of a series transformer according to claim 5, characterized in that, The step of issuing a trip command to the bypass switch to control the bypass switch to quickly disconnect, and formally connecting the series transformer in series into the main circuit between the power supply side and the load side, includes: Send a trip command to the drive circuit of the bypass switch; The bypass switch performs a disconnection action after receiving a trip command; After the bypass switch is disconnected, the secondary winding of the series transformer is no longer short-circuited. The series transformer is officially connected in series to the main circuit between the power supply side and the load side. The system topology is switched from normal bypass operation to series compensation operation. After synchronous detection confirms that the transformer has been normally put into operation and the main circuit topology switch is successful, a signal indicating that the transformer has been put into operation is sent to the upper-level controller.

10. The method for rapid connection of a series transformer according to claim 5, characterized in that, The inverter switches from current control mode to other control modes, and, in conjunction with the topology characteristics of the series transformer, adjusts and controls the electrical quantities of the main circuit to achieve the target functions required after the series transformer is put into operation, including: The inverter switches from current control mode to target control mode using a soft-switching strategy. The output value at the end of the current control mode is used as the initial setpoint for the target control mode, ensuring the inverter output voltage U is maintained during the switching process. inv No continuous mutations; After switching to the target control mode, the voltage analysis of the main circuit is performed using the topological characteristic of reverse connection of the same-name terminals of the series transformer. The load-side voltage U L With the effective value of the grid voltage U s Inverter output voltage U inv The following relationship must be satisfied: ； When it is necessary to compensate the load-side voltage to the target value Then, solve for the required output voltage setpoint of the inverter: ； by As the command value for the inverter output voltage, the closed-loop voltage controller drives the inverter output in real time, causing the load-side voltage U to... L Stable tracking target value ; During operation in the target control mode, the effective value of the grid voltage U is continuously monitored. S The load-side electrical quantities are measured, and when the triggering conditions disappear and the power grid returns to normal, the inverter output is controlled to return to zero smoothly. Then, the bypass switch is closed to re-short-circuit the secondary windings of the series transformer.

Images